Conventionally, there has been known a compressor which is used in a refrigerating machine and adapted for compressing refrigerant gas evaporated in an evaporator to convey the compressed refrigerant gas to a condenser (see, for example, Patent Document 1).
FIG. 6 shows a configuration of the compressor disclosed in Patent Document 1. The compressor disclosed in Patent Document 1 is a two-step-type screw compressor operable to compress the refrigerant gas including a chlorofluorocarbon or the like, in two steps. The compressor includes a pair of a first step screw rotors 101, 102, and a pair of second step screw rotors 103, 104. The respective screw rotors 101 to 104 are accommodated in a casing 106.
The first screw rotors 101 and 102 are so arranged as to engage with each other in a first compression chamber 106a in the casing 106. The second screw rotors 103 and 104 are so arranged as to engage with each other in a second compression chamber 106b in the casing 106. A rotor shaft of the each screw rotors 101 to 104 is supported by a corresponding bearing 108.
In the first compression chamber 106a, the first step screw rotors 101, 102 rotate with engaging with each other, and thereby perform a first compression of the refrigerant gas. The compressed refrigerant gas is introduced in the second compression chamber 106b. In the compression chamber 106b, the second screw rotors 103 and 104 rotate with engaging with each other, and thereby perform a second compression of the refrigerant gas. The refrigerant gas to which the second compression is applied is discharged from the compressor.
A lubricating oil is supplied to the respective bearings 108. A part of the supplied lubricating oil is included in the refrigerant gas and flows in the compressor, and is discharged from the compressor together with the refrigerant gas. The refrigerant gas and the lubricating oil discharged together are conveyed to an oil separator 110. The oil separator 110 separates the refrigerant gas from the lubricating oil. The separated refrigerant gas is conveyed to the condenser, while the separated lubricating oil is cooled by an oil cooler 111. The lubricating oil is then processed in an oil filter 112 which removes impurities in the separated lubricating oil, followed by being returned to the compressor to be again supplied to the respective bearings 108.
The above compressor requires the oil separator 110 for separating the refrigerant gas from the lubricating oil discharged from the compressor together with the refrigerant gas. Therefore, there is a problem that the configuration of the compressor becomes complex. In addition, in the above compressor, chlorofluorocarbon is used as the refrigerant gas. This raises a concern that a harmful effect, such as a global warming, is given to the natural environment on its disposal. Further, in the above compressor, the respective bearings 108 are lubricated with the lubricating oil. This results in a problem that a troublesome waste oil treatment is required upon the disposal of the waste oil.    Patent Document 1: Japanese Patent Application Publication No. 9-268988